Predicting Floods: The Flash Factor

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Flash floods are the single deadliest storm-related weather hazard, killing more than 140 people every year in the United States. Can we predict floods?

What are Flash Floods?

As the name implies, flash floods happen suddenly, leaving little time for warning (flash floods have developed in as little as 58 seconds). They usually occur on small streams with less than 20 square miles of drainage area. (“Regular” flooding may not occur until days or even weeks following a period of snowmelt or rain.)

A mere six inches of moving water can knock you off your feet, while two feet of water will carry away most cars.

Predicting a Flash Flood

To have any chance of predicting a flash flood, forecasters need to know where the “bullseye” of intense rain is—that is,

How much rain is falling,

how quickly it’s coming down, and

how saturated the soil is.

Radar and rain and stream gauges automatically convey this information via phone lines, radio waves, or satellites. “That way, the forecaster can receive [data] as the rain is falling,” says Matthew Kelsch with the Forecast Systems Laboratory, based in Boulder, Colorado.

However, radar is limited in its ability to detect rain in mountainous areas. In addition, the father away you get from the radar, the less accurate the information.

Another big unknown: How will the water behave once it’s on the ground? “Flash floods don’t always happen in the natural stream channel,” says Kelsch.

To address this, land surface characteristics such as terrain are now included in computer models along with data from radar and gauges, resulting in better predictions about how a certain location will handle a given amount of water.

The most difficult challenge is how to measure how much moisture the ground can soak up, NASA has begun exploring the use of satellites to measure soil moisture moisture over much larger areas than those currently monitored by rain gauges.

Flash Flood “Red Flags”

It may be difficult to predict a flash flood but you can keep your watch out for these factors:

Flash floods occur within six hours of a rain event.

Listen for news of dam or levee failures.

Watch for slow-moving thunderstorms that repeatedly moving over the same area.

Obviously hurricanes are another big source of intense rain.

If you see water is collecting in pools, this is a sign the ground is oversaturated with water.

During flood watches, do not park your car near a river or on a street that you know floods. As land turns from fields or woodlands into roads and parking lots, it loses its ability to absorb rainfall.

Be aware of a “flood watch” or “flood warning” from the National Weather Service.

Flood Watch

A Flood Watch is issued when conditions are favorable for flooding. It does not mean flooding will occur, but it is possible.

Flood Warning

A Flood Warning is issued when flooding is imminent or occurring.

For the Record

Forecast: Heavy rain, possible flooding

Fact: Thousands of campers in Colorado’s narrow Big Thompson Canyon ignored warnings of flooding on July 31, 1976, with tragic results. Twelve inches of rain fell in five hours, making the river rise with astonishing speed. A 19-foot wall of water killed 139 people, as it swept away boulders, cars, bridges, even a restaurant with the diners still inside.